Many of today's lightweight and/or high-speed engines, which are typically two-cycle engines, do not have a cam and valve overhead. When such engines are designed for fuel injection, means other than the cam must be used to actuate the injector. Addition of actuator components presents design problems, in that such components must not unduly increase the weight and cost of the engine. The actuation components must also be capable of withstanding the forces that will be exerted within the injection system due to high injection pressures and fast operation. If the actuation components require lubrication, this further increases the weight, cost, and complexity of the engine.
One alternative to a conventional cam-driven injection system is to use an injection pump mechanism external to the injector assembly. An example is a pump line nozzle injection system. However, there are limitations on where the injection pump can be mounted relative to the injector. This limitation is due to the compressibility to the fuel and the transport delay of the fuel pulse from the injection pump to the injector. This is an issue because of the high operating speed of these small engines.
Another alternative is a "unit injector", so called because their pumping mechanism is integral to the injector assembly. An example is a cam-actuated unit injector, such as that used in on-highway diesel engines.
However, for small two cycle engines, this type of cam actuation would present the weight and lubrication problems discussed above.